1. Field of the Invention
The present invention relates generally to CMOS image sensors, and more particularly to an apparatus and method for correcting digital column gain mismatch in CMOS image sensors.
2. Description of the Related Art
Visible imaging systems implemented using CMOS image sensors significantly reduce camera cost and power while improving resolution and reducing noise. The latest cameras use CMOS iSoC sensors that efficiently couple low-noise image detection and processing with a host of supporting blocks on a single chip.
However, CMOS imaging-Systems-on-Chip (iSoC) are prone to producing image artifacts of the analog readout architecture within the image. At each analog buffer stage, a finite offset and gain deviation may be introduced. The offset deviations result in coherent Fixed Pattern Noise (FPN) components such as “column FPN.” Depending on the amplitude, column FPN may become visible in low-light conditions when high gain must be applied. Pure digital corrections are often employed to align the offsets when the mismatches in the analog domain cannot be sufficiently controlled.
In general, whether the viewer is able to perceive coherent spatial or temporal noise depends on the contrast ratio to the random pixel noise. The ideal low-light scenario may be considered to be one in which each pixel has equivalent offset and response and purely Gaussian temporal noise. Any visible departure from that situation will degrade the perceived quality of the imaging system.
In the case of offset-matching, the most difficult environment is one of complete darkness. Here, the random pixel noise is minimized since there is no photon shot noise. It has been empirically determined that for uncorrelated Gaussian noise components (i.e. with a flat spatial frequency up to the Nyquist frequency), the amplitude of any coherent row-wise or column-wise element must be less than 1/10th of the random pixel noise, in order to appreciably degrade the image.
The situation for gain mismatch is somewhat different. In this case, the effect of darkness is not too much of a problem, the errors increase linearly with the number of integrated photons. The random pixel noise when light is present is dominated by Poisson fluctuations in the numbers of integrated photons. The magnitude of the photon shot noise is equivalent to the square root of the number of photons. Therefore, as more light is added, coherent (column-wise or row-wise) gain mismatches become harder to hide.
The most difficult scenario then for a given sensor is at the maximum possible photo-charge, i.e. the pixel full well. In order for the column-to-column gain mismatch to be invisible under all circumstances for a given sensor, a rule of thumb is that at full well, the column averages must have a sigma less than 1/10th of the photon shot noise.